The present invention relates to an automatic braking apparatus provided with an automatic braking control device for automatically actuating a friction braking device independently of a manually operated braking member, to apply a braking torque to each wheel, and particularly relates to the apparatus for performing the automatic braking control by the friction brake, when a restraining torque is being applied to the wheel with an engine brake, for example.
Recent vehicles are required to perform an automatic braking control, which is adapted to measure a distance from a vehicle to another vehicle ahead thereof or a difference between vehicle speeds of them, and adapted to perform a braking operation automatically to reduce the vehicle speed, if it is required to reduce the vehicle speed, and unless the vehicle driver is accelerating the vehicle, and which may be called as an adaptive cruise control (abbreviated as ACC). Now, control apparatuses for enabling the automatic braking control are getting popular.
For example, in Japanese Patent Laid-open Publication No. 11-268558, there is disclosed as a prior art, a braking and driving force control apparatus for measuring a distance between a vehicle and another vehicle ahead thereof, and controlling a vehicle speed, or braking and driving force, so as to maintain the distance to be of an appropriate value. Then, in order to improve a ride comfort of the vehicle and realize the braking and driving force control with a good responsibility in overall vehicle speed range, proposed is the braking and driving force control apparatus for controlling axle torque of driving wheels of a traveling control apparatus provided for controlling a distance between the vehicles, or controlling the braking and driving force. With respect to this apparatus, it is described that an engine torque command value is calculated in accordance with a braking and driving force command value, and a throttle opening command value for a throttle actuator is calculated on the basis of the engine torque command value and the number of rotations of the engine. Next, a lower limit value for the throttle opening command value is calculated to be variable in accordance with the vehicle traveling state, and the throttle opening is limited in accordance with the lower limit value. Next, based on the lower limit value for the throttle opening command value and the number of rotations of the engine, the engine torque is calculated, and modified value of the braking and driving force is calculated in accordance with the lower limit value of engine torque. Then, as it is so constituted that the command value of the braking and driving force and the modified value of the braking and driving force are input, to calculate amount of operation of a brake actuator, it is described that the lower limit of the throttle opening can be set in accordance with the vehicle traveling state.
According to the braking and driving force control apparatus as described in the Publication, it is aimed to obtain a desired deceleration, with the engine brake and friction brake being controlled coordinately. In this case, if the engine torque is varied, for example, a delay will be caused to reflect it to vehicle deceleration. However, it is difficult to compensate the delay with the friction brake. As for the torque to be transmitted from the power train including the engine to the wheels (driving wheels), there are a driving torque provided by the power train, and a torque provided by the engine brake or the like for acting in a restraining direction opposite to the driving direction, which is negative driving torque, and called hereinafter as restraining torque. However, it is difficult to estimate accurately the restraining torque. Therefore, it is very difficult to control the braking torque, which is provided for reducing the torque created on the wheel by the friction brake and the restraining torque as described above, coordinately. Yet, it is extremely difficult to do so only by the friction brake.
With respect to the torque applied to the wheels (driving wheels), the restraining torque corresponds to the torque provided in the direction for preventing the wheels from being rotated, as well as the braking torque. Therefore, both of the restraining torque and the braking torque result in the braking force. In this application, however, they are distinguished from each other, so as to identify their origins.